The present invention relates generally to tape cassettes, and more particularly, to the tape cassettes provided with a prism for detecting a tape end.
Referring to FIG. 14, a known tape cassette 101 of this type has a prism 103 as optical-path changing means mounted on the inner surface of a cassette or shell 102. When inserting the tape cassette 101 into a drive such as a tape recorder, an incidence plane 103a of the prism 103 faces a light emitting element 202 of a tape-position sensor 201 mounted to the drive, whereas a projection plane 103b of the prism 103 faces a light receiving element 203 of the tape-position sensor 201. Detection light 204 emitted from the light emitting element 202 enters the prism 103 through the incidence plane 103a, which is perpendicularly reflected by first and second reflecting surfaces 103c, 103d, then projected from the projection plane 103b to a magnetic tape 104 and a transparent leader tape, not shown, connected to an end thereof. A difference in quantity of detection light after passing through the magnetic tape 104 or the leader tape is detected by the light receiving element 203 to determine the tape end, etc.
Referring to FIG. 15, the cassette or shell 102, which is formed out of a thermoplastic synthetic resin, includes first and second bosses 111, 112 integrated with an inner surface of an upper half.
The bosses 111, 112 are arranged symmetrical with respect to a prism mounting reference line Cl.sub.1 which corresponds to an optical axis of detection light 204 emitted from the light emitting element 202.
The prism 103 is formed out of a transparent thermoplastic synthetic resin. The prism 103 includes horizontal and vertical portions 103e, 103f, having a substantially L-shape. Arranged on both sides of the horizontal portion 103e are first and second mounting base plates 121, 122 which are formed with first and second boss insertion holes 131, 132.
The boss insertion holes 131, 132 are arranged symmetrical with respect to an optical axis Cl.sub.2 of the horizontal portion 103e, each hole being shaped like a larger ellipse to enable engagement of the corresponding boss 111, 112 with predetermined clearances. Such shaping of the boss insertion holes 131, 132 allows easy and sure insertion of the bosses 111, 112.
The bosses 111, 112 are arranged through the boss insertion holes 131, 132, and ends of the bosses 111, 112 are put to a horn of an ultrasonic welding machine for softening and melting, obtaining heads 111a, 112a with larger diameter than that of the boss insertion holes 131, 132 as shown in FIG. 14. Thus, the prism 103 is mounted on the inner surface of the cassette 102.
With the known tape cassette 101, as described above, the bosses 111, 112 are arranged symmetrical with respect to the prism mounting reference line Cl.sub.1 which corresponds to the optical axis of detection light 204 emitted from the light emitting element 202, and the boss insertion holes 131, 132 are arranged symmetrical with respect to the optical axis Cl.sub.2 of the horizontal portion 103e of the prism 103. Moreover, predetermined clearances are defined between the bosses 111, 112 and the boss insertion holes 131, 132. As a consequence, referring to FIG. 16A, when making the optical axis Cl.sub.2 of the horizontal portion 103e correspond to the prism mounting reference line Cl.sub.1 or the optical axis of detection light 204 emitted from the light emitting element 202, and putting the bosses 111, 112 in contact with the inner peripheral surfaces of the boss insertion holes 131, 132, a first clearance .delta..sub.1 between the first boss 111 and the first boss insertion hole 131, and a second clearance .delta..sub.2 between the second boss 112 and the second boss insertion hole 132 are produced in the same direction of the bosses 111, 112 with respect to the optical axis or therebelow as viewed in FIG. 16A, having the following inconveniences:
First, it is difficult to make the optical axis Cl.sub.2 of the horizontal portion 103e of the prism 103 correspond to the prism mounting reference line Cl.sub.1. The reason is that, in the state as shown in FIG. 16A, if clockwise force F.sub.1 acts on the prism 103, the prism 103 is inclined clockwise as shown in FIG. 16B. On the other hand, if counterclockwise force F.sub.2 acts on the prism 103, the prism 103 is inclined counterclockwise as shown in FIG. 16C. This results in displacement of an optical axis of the prism 103.
FIG. 17 shows the relationship between the amount of displacement between the optical axis of detection light 204 emitted from the light emitting element 202 and that of the prism 103 and the return quantity of detection light 204. As seen in FIG. 17, if the amount of displacement between the optical axis of detection light 204 and that of the prism 103 is zero, the return quantity of detection light 204 is approximately 80% of the projection quantity thereof. However, if the amount of displacement is 0.2 mm, the return quantity is approximately 60%. If the amount of displacement is 0.3 mm or more, the return quantity is 50% or less, resulting in unstable and unreliable light detection.
Second, referring to FIG. 18, a width W of a detection-light window 141 of the cassette 102 can be reduced to decrease the amount of displacement. With this structure, if the prism 103 is inclined, an end of the horizontal portion 103e of the prism 103 contacts the inner side face of the detection-light window 141 of the cassette 102, restraining the amount of displacement. However, if ultrasonic wave is applied to the bosses 111, 112 in that state, the end of the horizontal portion 103e contacting the inner side face of the detection-light window 141 can be melted and deformed to have a bad influence on the performance of the prism 103.
It is, therefore, an object of the present invention to provide tape cassettes which enable stable and reliable light detection and easy assembling with simple structure.